The present invention relates to rolling bearings for use in vehicles, agricultural machines, construction machines, steel making machines, and the like. More particularly, the present invention relates to rolling bearings having a surface hardened layer with high impact resistance and long service life so that it is suitable for use in transmissions and engines.
Rolling bearings are used under severe conditions in which they are subjected to repeated shearing stress under high contact pressure. In order to withstand the applied shearing stress to thereby secure the necessary rolling fatigue life (hereinafter also referred to simply as "rolling life" or "life"), a high-carbon chromium bearing steel (SUJ 2) has generally been used as a bearing material. The steel is hardened and tempered to provide the Rockwell hardness of H.sub.R C 58 to 64.
Case hardening steels have also been used to extend the life of bearings. In order to set a hardness curve in accordance with the distribution of internal shearing stresses due to a contact pressure, low carbon case hardening steels such as SCR 420H, SCM 420H, SAE 8620H, SAE 4320H and the like, which have the superior hardenability, are carburized or carbonitrided, followed by hardening and tempering to produce inner and outer races and rolling elements that have the surface hardness of H.sub.R C 58 to 64 and the core hardness of H.sub.R C 30 to 48. Thus, the required service life has been secured by the above heat treatments.
However, no definite criteria have been set for determining the depth of a hardened layer that is appropriate for rolling bearings. For example, Unexamined Japanese Patent Publication No. Sho. 62-132031 has referred to the advances of the steel making technology in relation to the depth of the surface hardened layer in raceway rings and rolling elements of a rolling bearing. That is, it shows that the relationship between the depth of carburization in case hardening steels and service life which has drastically changed in the past ten years. Stated more specifically, the results of experiments conducted on case hardening steels in the 1970s showed that there was an optimal value for the depth of carburization and that the rolling life decreased irrespective of whether carburization was too shallow or deep. On the other hand, the results of experiments conducted on case hardening steels in the 1980s revealed that the rolling life elongated as the depth of carburization was increased. The results have been assumed to suggest the influence of non-metallic inclusions which serve as a source of stress concentration. On the basis of this assumption, Unexamined Japanese Patent Publication No. Sho. 62-132021 has proposed that the life of a rolling bearing extends by increasing the depth of the surface hardened layers in the raceway rings and the rolling elements to such a value as the depth relative to the diameter of each rolling element (depth/diameter) is 0.05 or more in the raceway rings and 0.07 or more in the rolling elements.
However, a surface hardened layer that is too thick not only increases the cost of heat treatments because an elongated time is required for carburization or carbonitriding but also deteriorates the superior impact strength property which is inherent from the surface hardening treatment.